1. Field of the Invention
The present invention relates to an optical disk apparatus which can write information to and read information from respective optical disks with different substrate thicknesses through the use of a single objective lens.
2. Description of the Related Art
Conventional optical disk drives form a laser spot on a disk by focusing a laser ray and radiating a laser spot on a recording layer of a disk. The laser spot is used to access the disk during reading and writing operations. The conventional optical disk apparatus records information by forming marks (pits) in a recording layer of the optical disk. In order to read information from the disk, the disk drive detects a reflective laser ray from the recording layer and reproduces information based on the content of the reflected ray.
The recording layer in the above-described disk lies between two transparent plates made of a plastic material. Recently, a high recording density disk called Digital Video Disk (DVD) has been developed. A DVD "high density" recording media requires transparent plates which are thinner than the those used for "standard density" disks. In "standard density" disks, the transparent plates are approximately 1.2 mm thick, in DVD and other "high density" disks, the transparent plates are approximately 0.6 mm thick. Although DVD is not the only type of "high density" disk, the present invention will be described with reference to DVD.
With the continued development of the "high density" disk, it has become desirable to be able to record and reproduce information on disks which have different transparent substrate thicknesses.
To reproduce information recorded on the recording layer properly, the laser ray which is focused on the "standard density" disk laser has about a 1.2 .mu.m diameter spot, and on the "high density" disk it is about a 1.0 .mu.m diameter spot. However, when a laser ray with the same wave length is focused on respective disks with different transparent substrate thicknesses by the same objective lens, a proper laser spot cannot be formed on the recording layer. In that case, a spherical surface aberration occurs. The spherical surface aberration is a defocusing, or spreading out of the normal laser spot. This defocusing degrades the quality of the signal which is being read from or written to the optical disk. Thus, there is an incompatibility between the "standard density" disk and the "high density" disk in that a single objective lens cannot access information from both.
The spherical surface aberration is caused by the change in the thickness of the transparent substrate. Due to the change in thickness, the circumference of the laser ray radiated on the recording layer is defocused, and the circumference of the reflective ray becomes noisy. In other words, instead of forming a well-defined circular spot on the recording layer, the circumference of the spot becomes wider and less well-defined. Accordingly, the quality of the reproduced signal is degraded.
The present inventor has proposed an optical disk apparatus having an objective lens designed to reduce the spherical surface aberration when recording and reproducing on a "high density" disk with a thin transparent substrate. Traditionally, if one wants to use an objective lens suitable for a "high density" disk to record and reproduce a "standard density" disk, an aperture is required. The aperture is a filter placed in the optical path between the objective lens and the recording layer of the optical disk, which corrects the defocusing problem discussed above. The aperture narrows the amount of light that can pass through to the optical disk and therefore creates a more well-defined laser spot on the recording layer of the optical disk. The aperture in essence avoids the defocusing problem caused by the difference in substrate thickness.
However, when recording or reproducing a "high density" disk in the above example, the aperture is unnecessary. Therefore, it is necessary to provide for a mechanism for inserting and removing the aperture from the optical path when the disks are changed. The mechanism for inserting and removing the aperture must be highly accurate. A circuit and mechanism which would control the aperture changing accurately is costly and difficult to manufacture. Further, vibrations of the spindle motor and the sledge motor are transferred to the aperture changing mechanism, causing further defocusing of the laser spot. Therefore, it becomes increasingly difficult to read and write information accurately. Hence, it is currently undesirable to use an aperture changing mechanisms in optical disk drives.